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Op-amp selection for switching current measurement

Kashyap Gundlapalli
Prodigy 110 points
Other Parts Discussed in Thread: TLV271

Hi All,

I am building a 800W pushpull converter operating at 40kHz - 65kHz switching frequency with primary current going till max 100A. Converter input voltage on primary side can vary from 30-42V. See the circuit shown in figure below.

I want to implement peak current mode control for this converter. So, I used a shunt resistor of 1mOhm to sense the primary switching current. Shunt can be placed in two positions (1 - high side of battery or 2 - low side of battery). 

In order to amplify the CS1-CS2 signal I used TLV271 op-amp (in case of low side sensing) but it is not able to reproduce the switching current signal. I need your help in finding the right op-amp for my application.

Thanks in advance!

Best Regards,

Kashyap Gundlapalli

  • 0
    TI__Guru* 93105 points

    Hello Kashyap,

    I believe there a couple of issues with your circuit:

    1. While the op amp's negative supply pin is referenced to ground, the transformer primary circuit floats. There is not ground return but through the op amp other than possibly the the input ESD diodes. The op amp placed across the high-side shunt is forced to a voltage level above the maximum positive VCM voltage, while the op amp placed across the low-side shunt is forced to a voltage below the maximum negative VCM. Therefore, the op amp won't function normally.
    2. The op amp does not have a feedback resistor, thus even if the circuit operated the gain would be at the open-loop gain (Aol) level.

    A special type of op amp based device called a current shunt monitor is designed to operate with VCM levels beyond the supply rails. I do believe you would still need to have a common ground between the primary circuit and the current shunt monitor devices. If you would like to view such devices you can find them here:

    www.ti.com/.../current-sense-analog-output-products.page

    If you decide to employ current shunt  monitors please note that support is provided at the Current Sensing E2E forum.

    Regards, Thomas

    Precision Amplifiers Applications Engineering